Varistors mainly comprised of silicon carbide (SiC), selenium (Se), silicon (Si), or zinc oxide (ZnO) have heretofore been used for the purpose of protecting electric and electronic devices from overvoltage. In particular, variators containing ZnO as a major component, which are described, for example, in U.S. Pat. No. 3,663,458, generally have advantages in that the limiting voltage is low and the voltage-nonlinear exponent is large. For this reason, they are suitable for use in the protection from overvoltage of electric or electronic devices comprising devices, such as semiconductors, having a low overcurrent withstand capacity and, therefore, have been increasingly used in place of varistors made of SiC.
It is known from Japanese Patent Publication No. 22125/81, Japanese Patent Application (OPI) Nos. 152205/81, 152206/81 and 152207/81, etc. (The term "OPI" as used herein refers to a "published unexamined Japanese Patent Application") that voltage-nonlinear resistors produced by adding a rare earth element, cobalt (Co), at least one element of potassium (K), rubidium (Rb), and cesium (Cs), and further chromium (Cr) in the form of element or compound to ZnO as a major component and sintering the resulting mixture are superior in voltage-nonlinearity. These voltage-nonlinear resistors, however, have disadvantages in that their short duration discharge current withstand capability is slightly low, and also their life performance under application of electricity are low. Thus, they give rise to problems in the miniaturization of elements.
The present invention is intended to elucidate the mechanism of breakdown of elements due to short duration discharge current and further to realize the inhibition of breakdown, and simultaneously to provide voltage-nonlinear resistors which have improved life performance under application of electricity, are of small size, and are superior in high short duration discharge current withstand capability and life performance under application of electricity.
It has been found that with conventional voltage-nonlinear resistors made of ZnO as a major component, and a rare earth element, Co, at least one element of K, Cs, and Rb, and Cr, when a large scale of short duration discharge current is applied, the concentration of electric field at the peripheral portion of an electrode provided on the surface of an element causes current concentration, and that the current concentration brings about the breakdown of the element. Furthermore, it has been confirmed that there are local uneven portions in the interior of the resistor, and it has been found that when a direct current is passed, the concentration of current at the uneven portions occurs, causing deterioration in characteristics.